The battery light comes on occasionally when driving; 7.3 liter diesel ford

Your PSD requires two strong batteries. Remove and fully charge both batteries, adding distilled water if necessary. Most auto parts stores will charge batteries for free.

Afterwards, check the overall health of the charging system with a multimeter:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-10.html#post1973613

The battery light coming on may indicate the brushes in the alternator are worn and need replacing--easy job on some Motorcraft alternators.